1. Field Of The Invention
The present invention relates to an apparatus usable for ionizing a gas and comprising a cathode serving as a hot or cold cathode as well as to a process for using said apparatus.
The invention applies to all apparatuses used more particularly for ionizing a gas, such as electric arcs, unoplasmatrons and duoplasmatrons. For reasons of clarity of the description, the invention will be described on the basis of a duoplasmatron, e.g. used in surface analysis equipment as an ion source for abrading samples. In known manner, a duoplasmatron comprises either a cold cathode, or a hot cathode.
2. Background Of The Invention
FIG. 1 diagrammatically shows in longitudinal section a cold cathode duoplasmatron of a known type. this duoplasmatron comprises a cylindrical hollow cathode 1, whose upper part is mounted on a generally conductive support 2, an intermediate electrode 3 surrounding cathode 1 and having in its lower part an opening 4 and an anode 5 surrounding the intermediate electrode 3 and provided with an outwardly divergent opening 6 facing the opening 4 of said electrode.
In general, the cathode is made from nickel and the intermediate electrode 3 and anode 4 are made from soft iron.
The terms "upper part" and "lower part" of each element are defined in this text relative to the displacement direction of the gas to be ionized through the duoplasmatron.
Cathode 1 mounted on support 2, intermediate electrode 3 and anode 5 are electrically insulated. These three components are located within one another so as to define three intercommunicating chambers 11, 13, 15, joints 8, 9 ensuring the sealing of said chamber with the exterior. Chamber 11 is defined by the inner cylindrical walls of the cathode, chamber 13 by the space between cathode 1 and intermediate electrode 3 and chamber 15 by the space defined between intermediate electrode 3 and anode 5.
Moreover, a magnetic coil 21 surrounds chambers 11, 13, 15. This coil is located around the upper part of anode 5 and rests both on the lower part of anode 5 and the upper part of intermediate electrode 3.
Moreover, a voltage generator 23, e.g. connected to the lower part of anode 5 and to the conductive support 2 of cathode 1 makes it possible to apply a potential difference Va-vc of approximately 300 to 500 V between the anode and the cathode, Va representing the voltage applied to the anode and Vc the voltage applied to the cathode. Moreover, a voltage generator 25 is connected to the intermediate electrode, e.g. to the upper part of said electrode and to ground. Voltage generator 25 makes it possible to apply a voltage Vi to the intermediate electrode, said voltage Vi being generally such that Vi=(Va-Vc)/2. Voltage Vi can also be obtained from the voltage generator 23 via a divider bridge connected to the intermediate electrode and to the voltage generator 23 and in this case generator 25 is eliminated.
Not shown vacuum forming means, such a vacuum pump, ensure the discharge, e.g. via opening 6 of all gases present in chambers 11, 13 and 15 prior to the introduction of the gas to be ionized into the duoplasmatron.
The gas to be ionized is e.g. stored in a cylinder 16 connected by a pipe 17 to support 2 of cathode 1, said support having a passage 10 connected to chamber 11. Opening and closing means, such as a valve 17' e.g. located on pipe 17 makes it possible to introduce a regulated gas flow into the duoplasmatron from cylinder 16.
The remainder of the description makes it possible to understand the operation of the cold cathode duoplasmatron.
Before forming a vacuum in chambers 11, 13 and 15, gas is introduced into the duoplasmatron by opening valve 17'. The gas circulates in chamber 11, where it will be ionized by the electrons emitted by cathode 1, to which potential Vc is applied. A plasma of ions and electrons then forms and will be moved toward the intermediate electrode 3 by the electric field E.sub.1 induced by the potential difference Vi-Vc between cathode 1 and electrode 3. This plasma will pass through the opening 4 moved by an electric field E.sub.2 induced by the potential difference Va-Vi between electrode 3 and anode 5, as well as by a magnetic field H between electrode 3 and anode 5.
Field H circulates in a closed loop between magnetic coil 21, intermediate electrode 3 on which the coils rests, the part of chamber 15 defined between the lower part of the intermediate electrode 3 and the upper part of anode 5 and finally anode 5, on which the magnetic coil also rests.
Thus, the plasma is defined by the electric field E.sub.2 and the magnetic field H between the intermediate electrode 3 and anode 5. This plasma then passes through the opening 6 made in anode 5, moved by an electric field E.sub.3 induced by the potential difference between the anode and the generally grounded surface 20 to be abraded. These electric and magnetic fields E.sub.1, E.sub.2, E.sub.3 and H have the same direction and sense as the gas flow.
In order to function as a hot cathode, this duoplasmatron must be disassembled and the cylindrical cathode 1 replaced by a filament helically wound in accordance with a cylinder, whose axis is perpendicular to the gas flow direction. This filament is connected by each of its ends to a separate conductive tab or clip mounted on support 2, the gas to be ionized passing between said tabs and through the filament.
As hereinbefore, voltages Vc, Va and Vi are respecitvely applied to support 2, anode 5 and intermediate electrode 3. Moreover, a voltage Vs is applied between the two ends of the filament by a voltage generator. Said voltage Vs permits the flow of a current I in the filament and consequently the heating of the filament by the Joule effect.
For a cold cathode, the electron emission is more particularly produced by the bombardment thereof by the ions and electrons of the plasma. Thus, this emission is dependent on the conditions prevailing in the duoplasmatron, such as the pressure, electric fields and the nature of the gas. The electron emission of a hot cathode, due to the heating of the filament, in particular determined by its temperature and therefore the intensity of the current I, which can easily be regulated. Thus, the hot cathode permits a more stable electron emission, whereof it is possible to regulate the generally higher intensity than that supplied by a cold cathode. Therefore, when functioning as a hot cathode, a potential difference Va-Vc of a few dozen volts is adequate.
As a function of the type of gas to be ionized, it may be more advantageous to use either a cold cathode or a hot cathode. The cold cathode is used with reactive gases such as oxygen, which would attack a filament (hot cathode), whereas for inert gases such as argon, xenon, etc, it is more advantageous to use a hot cathode. Thus, the hot cathode makes it possible for the duoplasmatron to operate with a lower pressure of approximately 10.sup.-4 Pa and, as the electron emission for ionizing the gas is more stable, gives a better stability of the stream of ions extracted.
With a known duoplasmatron, for passing from hot cathode operation to cold cathode operation, it is necessary to disassemble the duoplasmatron, which makes it necessary to stop the operation thereof and break the vacuum.